While professor of medicine (1657–98) at the University of Copenhagen, Bartholin observed that images seen through Icelandic feldspar (calcite) were doubled and that, when the crystal was rotated, one image remained stationary while the other rotated with the crystal. Perceiving that light passing through calcite was split into two rays, he called the stationary image the “ordinary beam” and the moving image the “extraordinary beam.” Although Bartholin himself was unable to explain double refraction, it was recognized as a serious contradiction to Isaac Newton’s optical theories.

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an optical property in which a single ray of unpolarized light entering an anisotropic medium is split into two rays, each traveling in a different direction. One ray (called the extraordinary ray) is bent, or refracted, at an angle as it travels through the medium; the other ray (called the...

...properties of which differ along two or more of the crystal axes. What is observed depends on the angle of the beam with respect to the entrant face. Double refraction was first observed in 1669 by Erasmus Bartholin in experiments with Iceland spar crystal and elucidated in 1690 by Huygens.

In 1669 another Danish scientist, Erasmus Bartholin, discovered the polarization of light by double refraction in Iceland spar (calcite). This finding had a profound effect on the conception of the nature of light. At that time, the only waves known were those of sound, which are longitudinal. It was inconceivable to both Newton and Huygens that light could consist of transverse waves in which...